The following relates generally to devices, systems, and methods used to determine the distance between facial features, particularly the interpupillary distance.
Proper prescription and sizing of corrective eyeglasses requires measurement of several biometric dimensions of the face and optical system of the patient. Personalized lenses are crafted not only using the amount of corrective power required to assist the patient's eyes in focusing on objects, but also using precise measurements of the patient's facial features, including the distance between the centers of each of the pupils of the patient's eyes, also known in ophthalmological terms as the interpupillary distance. The distance is formally defined as the distance from the optical axis of one eye to the optical axis of the other eye, where the optical axis extends through the center of each pupil normal to the surface of the eye. Knowing this distance, the center of the corrective lens can be oriented directly in front of each eye along these axes, leading to clearer and more comfortable vision. An incorrect measurement of the interpupillary distance (even by two millimeters) creates a misalignment of the ophthalmic lenses relative to the eyes, leading to imperfect vision and eye strain.
Other biometric data used in lens design include the distance from the patient's nose to the optical axis of the eye, known as mono-pupillary distance, and the distance from the center of the pupil to the lens, referred to as the vertex distance. The inclination angle between the direction of the lens and the optical axis may also be significant in providing healthy vision correction. The interpupillary distance may also be determined depending on the position of the pupils at near or far vision, which may be referred to as the near interpupillary distance or the far interpupillary distance.
Existing systems and methods for measuring biometric data such as the interpupillary distance are complex and costly, and are mainly only usable by specialized, trained technicians, optometrists, and ophthalmologists in clinics or other offices. The systems used are quite accurate, measuring distances with error of less than 1 millimeter, but use specialized, sophisticated machinery and often require extra objects to be placed above the patient's face or over the frame. They are also expensive to use, requiring the active participation and time of these trained specialists to achieve the desired results and requiring the patient to travel to the office or clinic to gather the measurements.
To address some of these issues, various systems and methods have been presented to use cameras and other common devices to measure the patient's interpupillary distance remotely and without need for interaction of a trained specialist. For example, systems have been devised where one or more objects with predefined markers such as ruler markings are placed on or around the face and a sequence of images of the face and objects are captured and compared to each other to determine the necessary dimensions. However, in these systems the presence of the marker objects is crucial to obtain a geometric relationship between the pictures, and the user is expected to take pictures of himself or herself in various predefined poses which may be difficult to obtain alone while maintaining accuracy of the measurement. It is common for these methods to produce measurement error of 3 millimeters or more due to parallax errors and due to misuse of the marker objects, so they are generally discouraged by optometrists.
In other systems, no artifact or object is required on or around the face. Instead, the patient makes a predetermined set of movements, and the system applies a statistical calculation which is based on an iterative optimization algorithm run on the set of images gathered for estimating the distance to the camera, the interpupillary distance, the focal length of the camera, the radius of the eyeball and pupil sizes. The effectiveness of these systems is limited, since there is a computational burden of analyzing the images of the patient's movements, the patient's movements can be difficult to control. Furthermore, these systems often rely on automatic detection of the circle around the pupil, which can be difficult since that relies on sophisticated methods of detection and a high camera resolution.
In recent years, the market for online sale of ophthalmic lenses and frames has increased significantly, and with this new market there is a need for accurate, remote detection of the interpupillary distance. Existing methods of determining the interpupillary distance may be too expensive, too inaccurate, or too unreliable, or require the patient to go to an ocular specialist, thereby defeating a main advantage of online sales—buying without the physical presence of the customer. Thus, there is a need for an accurate, inexpensive, simple system that can be used by patients (to obtain the patient's interpupillary distance) without need of specialized artifacts or specialized technicians in this market and others.